The present invention relates to a method for the preparation of an organopolysiloxane ethylenically polymerizable only at a single molecular chain end or, more particularly, to an efficient method for the preparation of an organopolysiloxane having a straightly linear molecular structure of a controllable molecular weight or molecular weight distribution and having an ethylenically unsaturated polymerizable linkage only at one of the molecular chain ends.
As is known, organopolysiloxanes having ethylenical polymerizability only at a single molecular chain end are widely used in the polymer industry because a copolymer of such an organopolysiloxane and (meth)acrylic acid or an ester thereof as well as other vinyl monomers is imparted with various desirable properties such as water-repellency, heat resistance, weatherability, abrasion resistance and the like.
A method for the preparation of such a polymerizable organopolysiloxane is disclosed in Japanese Patent Publication No. 59-78236, according to which a cyclic organopolysiloxane oligomer such as hexamethyl cyclotrisiloxane is subjected to a ring-opening living polymerization in the presence of an anionic polymerization initiator such as organolithium compounds, alkali metal hydroxides, alkali metal alkoxides and alkali metal silanolates and the thus obtained organopolysiloxane is subjected to a desalinating condensation reaction with an .omega.-(meth)acryloxyalkyl dichlorosilane.
In this method, however, formation of hydrogen chloride is unavoidable because the chlorosilane reactant must be used in an excess amount over stoichiometry so that the hydrogen chloride acts as a catalyst to promote the siloxane rearrangement equilibrium reaction as a consequence of scission of the siloxane linkages leading to broadening of the relatively narrow molecular weight distribution of the organopolysiloxane as obtained by the living polymerization.